1. Field of the Invention
The present invention relates in general to electrolytes and, in particular, to solid electrolytic perovskites that have ion conductivities greater than 10−5 S/cm in a temperature range of 0–400° C.
2. Description of Related Art
Electrolytes play a fundamental role in areas as different as transmission of nerve signals in living organisms to the generation of electricity from chemicals in batteries. Electrolytes are able to play a fundamental role in these areas, because they have properties including, for example, rapid motion of at least one ionic species, comparatively low electronic conductivity, and a large electrolytic domain. Rapid ion motion is most commonly associated with liquid electrolytes, however, several solid electrolytes have been observed to have rapid ion motion. There has been considerable interest in the use of such solid electrolytes in fuel cells, sensors, batteries, steam electrolysis, membrane reactors etc. because they have several advantages over liquid electrolytes. For example, solid electrolytes form a more effective barrier between electrodes, do not leak, evaporate, or flood, and are less prone to parasitic electrode reactions than liquid electrolytes. Unfortunately, traditional solid electrolytes have not yet achieved wide-spread use in industry because they have or are: (1) low ionic conductivity (<10−5 S/cm) even at temperatures as high as 600° C.; (2) expensive or difficult to synthesize and process; (3) small electrolytic domain; (4) high raw material costs; and (5) chemically unstable. Table #1 lists the room temperature ionic conductivity for some solid electrolytes, and the issues or problems preventing their usage.
TABLE #1MATERIALCONDUCTIVITY (S/cm)ISSUES/PROBLEMSProton ConductorsNafion2 × 10−2Operating temperature limited to<90° C. due to water loss.H3OUO2PO4H2O4 × 10−3Radioactivity, expensive, water lossat temperatures >120° C.Doped BaCeO310−5 (500° C.)Unstable with respect to CO2 and H2O,rapidly decomposes.HNbO35 × 10−8Low conductivity, difficult to form amembrane.NH4-β-Al2O34 × 10−4Two-dimensional conduction path,difficult to synthesize.Lithium Ion ConductorsLiI-35 mol % Al2O34 × 10−5Conductivity to low for most batteryapplications.Li3N4 × 10−4Small electrolytic domain, unstable inair.Li2S—SiS2—Li3PO47 × 10−4Deliquescent in air.Li1.5Al0.5Ge1.5P3O122.4 × 10−4Germanium is too costly, grainboundary limited.Li0.33La0.55TiO32 × 10−5Ti reduced by Li metal, grain boundarylimited.
Three notable devices that have utilized traditional solid electrolytes are cardiac pace-maker batteries, oxygen sensors for automobiles, and electrochromic windows. However, opportunities exist both for the improvement of traditional electrolytic solids and for the development of entirely new solid electrolytes. For example, in the area of energy production, fuel cells are considered to be an attractive alternative to diesel and coal-fired power plants for the production of electricity since they are inherently clean, more efficient, quieter, etc. The two fuel cell technologies that are viewed with the most optimism for eventual commercialization are solid oxide fuel cells (SOFC) and polymer electrolyte fuel cells (PEFC). SOFC's based upon a stabilized zirconia electrolyte must be operated at temperatures in the range 750–1000° C. to achieve sufficiently high oxygen ion conductivity. Materials capable of withstanding these high operating temperatures are costly and difficult to fabricate. As such, SOFC designers have focused on lowering the minimum operating temperature to ˜650–750° C. so that cheaper electrocatalysts, seals, and interconnects may be used. On the opposite extreme, PEFC's utilize a proton conducting water-swelled polymer such as the one developed by Dow Chemical Company and sold under the trade name Nafion® that cannot be used at temperatures higher than ˜90° C. (see Table #1). Because, water evaporates from the polymer at higher temperatures, and proton conductivity falls by several orders of magnitude. Fuel cell electrodes are sensitive to operating temperature, as well. Anodes of SOFC's, due to the high operating temperature, can burn reformed hydrocarbon fuels that contain CO and are resistant to poisoning by sulfur containing compounds. On the other hand, PEFC's anodes are rapidly poisoned by CO at 90° C. and are limited to clean H2 as fuel. As such, a fuel cell based upon a solid proton conducting electrolyte that operates in the temperature range 200–400° C. would combine the best features of SOFC's and PEFC's, but no solid electrolyte with suitable proton conductivity and chemical stability exist in the market place prior to the present invention.
There is also strong demand for cheap, rechargeable, high power and energy density lithium ion battery. Sony offers a rechargeable lithium ion battery, but it is expensive. The Sony battery is based upon a liquid LiPF6 electrolyte that is toxic, corrosive, and pyrophoric in air. The recharging processes is delicate, and every Sony battery contains an electronics package which monitors and prevents overcharging that could lead to a fire. An alternative to the Sony battery is sought after, but there are no solid lithium ion conductors in the market place prior to the present invention with the right combination of materials properties and low cost. The aforementioned potential applications alone are enough to establish that there is a need and commercial value in development of new solid electrolytes. These needs and other needs are satisfied by the electrolytic perovskite, the solid proton conductor and the method of the present invention.